Chapter 1: Biology – The Study of Life (Mini-Textbook Study Notes)

Biology: The Study of Life

1.1 What Does It Mean to Say That Something is Alive?

Biologists define life by a set of five fundamental characteristics shared by all living organisms. Understanding these properties helps distinguish living things from non-living matter.

• Cells: All organisms are composed of membrane-bound cells, which are the basic units of life.

• Replication: All organisms are capable of reproduction, ensuring the continuation of their species.

• Information: All organisms process hereditary information encoded in genes and respond to information from their environment.

• Energy: All organisms acquire and use energy to maintain their internal order and support life processes.

• Evolution: Populations of organisms are continually evolving, adapting to their environments over generations.

Example: A bacterium, a tree, and a human all meet these five criteria, though they differ greatly in complexity.

Theories in Biology

In science, a theory is an explanation for a broad class of phenomena or observations, supported by a substantial body of evidence. This differs from the everyday use of "theory" as a mere guess.

• Cell Theory: What are organisms made of? Where do organisms come from?

• Theory of Evolution by Natural Selection: How are organisms related to one another?

• Chromosome Theory of Inheritance: How is hereditary information transmitted from one generation to the next?

1.2 Life is Cellular and Replicates through Cell Division

Discovery of Cells

The invention of the microscope allowed scientists to observe cells for the first time, leading to the development of cell theory.

• Robert Hooke (1665): Used a 30x microscope to observe small compartments in cork, which he called "cells."

• Anton van Leeuwenhoek: Improved magnification to 300x and observed single-celled organisms ("animalcules").

• By the 1800s, biologists recognized that all organisms consist of cells.

Cells: Highly organized compartments separated from their environment by a membrane barrier.

Cell Theory and Spontaneous Generation

• Cell Theory: All organisms are made up of cells, and all cells come from preexisting cells.

• Spontaneous Generation Hypothesis: The belief that organisms could arise spontaneously under certain conditions.

• All-cells-from-cells Hypothesis: Cells are produced only when pre-existing cells grow and divide.

Louis Pasteur’s Experiment

Pasteur tested whether cells arise spontaneously or from preexisting cells using nutrient broth in two types of flasks (straight-necked and swan-necked). Only the straight-necked flask, which allowed airborne cells to enter, showed cell growth. This supported the all-cells-from-cells hypothesis.

Life Replicates through Cell Division

• Cells must replicate for life to exist.

• All cells in a multicellular organism descend from preexisting cells, sharing a common lineage.

• Evidence suggests life arose from non-life via chemical evolution early in Earth's history.

1.3 Life Processes Information and Requires Energy

Chromosome Theory of Inheritance

• Proposed by Sutton and Boveri: Hereditary information is encoded in genes located on chromosomes.

• By the 1950s, chromosomes were identified as molecules of deoxyribonucleic acid (DNA).

• Genes are segments of DNA that code for cell products.

Structure of DNA

• James Watson and Francis Crick: Proposed that DNA is a double-stranded helix.

• DNA consists of a backbone and paired bases (A, T, C, G).

The Central Dogma

• Describes the flow of genetic information in cells:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• DNA codes for ribonucleic acid (RNA), which codes for proteins.

• Proteins determine an organism’s physical traits.

Genetic Information and Variation

• DNA is copied to pass genetic information from cell to cell or to offspring.

• Copying is highly accurate, but mistakes (mutations) can occur.

• DNA sequence changes may lead to changes in proteins, resulting in heritable variations and diversity of life.

Energy and Nutritional Needs

• Chemical reactions inside cells require energy.

• Organisms have two fundamental nutritional needs:

  • Acquiring chemical energy in the form of adenosine triphosphate (ATP).

  • Obtaining molecules used as building blocks for DNA, RNA, proteins, etc.

• The way organisms acquire energy is central to the diversification of life.

1.4 Life Evolves

Evolution and Natural Selection

• Evolution: Change in the characteristics of a population over time; species are related and can change through time.

• Population: Group of individuals of the same species living in the same area at the same time.

• Darwin and Wallace: Proposed that species are related by common ancestry and that characteristics can be modified from generation to generation (descent with modification).

Natural Selection

• Explains how evolution occurs.

• Two conditions for natural selection:

  1. Individuals vary in heritable characteristics.

  2. Certain heritable traits help individuals reproduce more successfully in particular environments.

• Natural selection acts on individuals, but evolutionary change occurs in populations.

• Speciation: Occurs when populations diverge to form new species.

Fitness and Adaptation

• Fitness: The ability of an individual to produce offspring. Individuals with higher fitness leave more surviving offspring.

• Adaptation: A trait that increases fitness in a particular environment.

• Example: On the Galápagos Islands, finches with small, pointed beaks had higher fitness when small, soft seeds were abundant. This adaptation increased their population frequency.

1.5 The Tree of Life Depicts Evolutionary History

Phylogeny and the Tree of Life

• The tree of life is a family tree describing genealogical relationships among species, with a single ancestral species at its base.

• Phylogeny: The actual genealogical relationships among all organisms.

Analyzing Genetic Variation

• Biologists compare RNA and DNA sequences from different organisms to analyze genetic variation.

• Fewer sequence differences indicate a closer evolutionary relationship.

• Example: Green algae DNA is more similar to land plant DNA than to brown algae DNA, indicating a closer relationship.

The Phylogenetic Tree of Life

• A phylogenetic tree shows relationships between species based on genetic data.

• Branches sharing a recent common ancestor represent closely related species; distant branches represent more distantly related species.

Interpreting the Tree of Life

• The tree of life indicates three major groups of organisms:

  • Eukaryotes (have a nucleus): Eukarya

  • Prokaryotes (lack a nucleus): Bacteria and Archaea